Synthetic polymer-based scaffolds have been successfully utilized to assist in the healing process of wounds unable to heal on their own. In such wounds, called critical sized defects, the appropriate cells fail to infiltrate sufficiently into the wound site and create the tissue that was lost in the injury. Compounding this problem is the lack of sufficient biological cues, such as growth factors, cytokines, and hormones that induce endogenous cells to migrate into injury sites, proliferate, and differentiate into all of the tissue types needed to repair the wound. Instead of the appropriate tissue replacing the lost tissue in critical sized defects, inflammatory cells and fibrotic cells infiltrate the injury site and form scar tissue that, if left in place, will act as a barrier and is likely to permanently prevent tissue regeneration.
The use of scaffolds to facilitate healing of critical size defects is provides multiple benefits. Scaffolds provide structural support to an injury site. Such structural support is essential as structurally deficient voids have a tendency of prolonging the injury response and often lead to further injury when stress, such as that caused by patient movement, is placed on the injury site. Scaffolds can also be fabricated to contain an interconnected inner porous structure where the pores provide a bridge for cells to infiltrate throughout the scaffold. The proximity of the scaffold with the injured tissue helps ensure that the appropriate cell types enter the structure. Additionally, as the scaffold degrades over time newly formed tissue can take its place and provide structure to the affected area. Further, as a compliment to enhancing appropriate regeneration, scaffolds can also act as a physical barrier to inappropriate cell types, such as macrophages and fibrotic cells, from infiltrating and forming a scar. The slowing of scar formation is essential because the regenerative process is generally in competition with scar formation.
While scaffolds alone have been found to help with the healing of increased size defects, further enhancement appears to be necessary to effectively heal large defects. In the case of craniofacial bone reconstruction, a critical size defect in the rabbit calvaria is 15 mm. Optimization of the scaffold composition, porosity, and degradation can enhance regeneration but the scaffold alone fails to promote the formation of bone throughout the defect area. Consequently, there is a need to introduce biological cues that can aide in the regenerative process.
Proteins, such as cytokines and growth factors, are potent inducers of cellular activity and as such have tremendous potential to be combined with scaffolds to achieve optimal regeneration. Predicate devices for bone fusions, such as Medtronic's Infuse™, Mastergraft™, and Amplify™ are mixtures of recombinant human bone morphogenetic protein (rhBMP-2) with bovine collagen sometimes in a ceramic scaffold. Recombinant human platelet derived growth factor-BB (rhPDGF-BB) is another protein with osteoinductive capability, and has been mixed with a ceramic by Biomimetic Inc. and marketed as Augment Bone Graft™ to aid in bone fusions of the ankle and foot. Several other such devices exist, all being variations upon the theme of mixing rhBMP-2 or rhPDGF-BB with collagen or a ceramic material. The effects of these devices are well established for their on-label uses for bone fusions, most often aimed toward the spinal cord. Off-label uses of these items, although technically prohibited, are thought to be common for applications that currently have no approved devices.
In the previously mentioned devices, the cytokines are mixed with the other components of the devices and are not specifically bound. The inability to specifically tether or bind the proteins to the devices likely necessitates using more rhBMP-2 and rhPDGF-BB than is actually necessary to achieve the desired cell and tissue responses. As the devices take up water and begin to erode/degrade, the rhBMP-2 and rhPDGF-BB will be released into the surrounding tissue. Good manufacturing practice (GMP) production of rhBMP-2 is thought to exceed $50,000 per milligram, making the use of excess rhBMP-2 or rhPDGF-BB very costly.
A possible solution to this issue is to tether the biomolecules (such as rhBMP-2 and rhPDGF-BB) to the scaffold. Such tethering would prevent the biomolecules from being wasted by diffusing out of the injury site while still allowing the biomolecules to interact with cell surface receptors to induce the desired biological response. The prolonged presence of the biomolecules, due to prevention of the diffusing away of the biomolecules, would enable a longer time period for inducing biological responses. This greatly decreases the quantity of the biomolecules needed to induce the desired cell and tissue responses, and hence the cost of the treatments. Lowering the costs of these treatments further allows for their expanded use in a clinical setting.
There has been much effort to develop a device with tethered biomolecules, yet no predicate devices exist. This is due to a combination of the following issues: (i) inefficiency of the tethering reaction of the biomolecule with the scaffold, (ii) slow speed of tethering reactions of biomolecules to the scaffolds, (iii) high cost of GMP biomolecules, particularly recombinant proteins, (iv) reduced potency of biomolecules that are tethered versus free, (v) difficulty tethering biomolecules uniformly throughout scaffolds, (vi) incompatibility of polymer solvents with biomolecules, particularly recombinant proteins, and (vii) inflexibility of tethering platform to easily adapt to a variety of biomolecules and scaffold devices. While some of these issues, such as cost of GMP proteins, are unavoidable, many of the remaining issues must be addressed in order to field a viable device that utilizes a protein tethering strategy.